Seamless cans such as are formed by a drawing and ironing process are widely used for soft drink containers, beer containers, etc. While can manufacturers prefer to form the cans with as little material as necessary to save on costs, certain requirements must be met for the safety of consumers. The cans must be able to withstand both external and internal pressures that may be encountered during storage and use. For example, cans used for soft drinks and beer must be able to withstand external pressure from stacking as well as internal pressure from the liquids therein due to temperature and atmospheric pressure changes as well as carbonation.
One important test that can ends must be able to withstand is a buckle test which is a test of the strength of the components of the can end to resist internal pressures and requires that a can end withstand a pressure change from 0-90 psi in 20 seconds or less without failure. Failures can occur at the neck of the can, along a seam of the can or along the interface between the pull tab and the can top. One known device for testing buckle strength uses an empty but sealed can which is inserted into a holder and then placed within a box. A lid to the box is closed over the can which simultaneously punctures the side of the can with a hollow needle. Air is then pumped through the needle into the can until the can end buckles (or some other failure occurs, such as a seam failure or dome reversal). Due to the design of the holder, some support is provided for the structure of the can by contact therewith. Thus, although not quantified, the tester may not provide an accurate indication of the actual buckle strength of the can end.
Additionally, puncturing the side of the can tends to form a crease or deflection in the side of the can due to the typically long, thin and unsupported sidewalls. Some air must be pumped into the can just to force the can to redeflect and seal around the puncturing device. Creasing the can with the needle also causes a weakening of the can which may tend to distort any results of the test. Additionally, due to the structure of the prior art tester, there is a potential for injury to an operator from projectiles (pieces of the can) escaping the box and from extremely loud explosions when the can fails at a seam.
Another testing apparatus is disclosed in U.S. Pat. No. 3,958,448 to Willis et al., May 25, 1976. The Willis device is primarily designed for pressure testing bottles by puncturing a cap on the top of the bottle. In operation, the bottle is placed on a stand in an up-side-down orientation followed by the puncturing of the cap by a hollow needle. A container retaining member limits upward movement of the bottle by placing a cross-bar over the bottom end thereof. The retaining member is positioned by an adjustable slide arrangement to retard movement of the bottle off the needle. The bottle is first placed in the apparatus and the top is manually punctured by pushing the bottle down onto the needle. The container retaining member is then adjusted to provide a desired gap between the end of the bottle and the cross bar. Thus Willis is primarily a manually operated device that requires repeated readjustment of the container retaining member for various container sizes.
Another testing apparatus is disclosed in U.S. Pat. No. 4,555,935 to Elert, Dec. 3, 1985. The Elert device uses a hollow needle to puncture a side wall of the container. Thus the Elert device is subject to the same disadvantages as the Reynold's tester as previously described above. Specifically, puncturing the side of the container tends to deform and weaken the container at that point. Additionally, a holder is provided which tends to strengthen and support the sides of the can where contacted thereby. Inaccurate results may be obtained from the Elert tester for the same reasons as with the prior art tester previously described above. Thus, there is a need for a method and apparatus for conducting a buckle test on a container which is convenient, easy and safe to operate, and provides consistently accurate results.